In a diffusion furnace for the processing of semiconductor wafers, a batch of wafers is introduced into the diffusion tube of the furnace and is subjected to a cycle composed of several intervals of time at predetermined temperatures and gas concentrations. Control of such diffusion processes have been accomplished by semi-automatic means whereby time, temperature and gas flow for each of a succession of time intervals are preset and maintained by known open and closed loop control circuits. Such semi-automatic control techniques are not very versatile and are often limited in the precision of control which can be achieved. Automatic control of diffusion processes has been proposed using a general purpose computer in association with a large number of diffusion tubes to provide, on a time-shared basis, monitoring and control of the several tubes in the overall diffusion facility. The use of a general purpose computer for control of less than a relatively large number of diffusion tubes is not economically realistic in most instances. By reason of the time sharing of a single computer, some process parameters cannot be readily controlled to the required extent. Moreover, any malfunction or interruption of the single computer can result in shutdown of the entire multiple tube diffusion facility, with serious consequences to the production of semiconductor wafers.